1. Field of the Invention
The invention in general relates to systems which utilize light scattering principles to detect and count undesirable single particles in fluids, referred to in the art as light scattering particle counters, and more particular to such a particle counter that utilizes a laser diode light source.
2. Statement of the Problem
The principles of light scattering are widely used for detecting and analyzing particles in or of a fluid. The present invention relates to the science of utilizing the principles of light scattering to detect and measure the size of individual particles suspended in a fluid. Each particle that is detected is counted, and an indication of the number of particle counts within a channel, with each channel corresponding to a particular size range, is provided. For particle counters to operate effectively, the density of particles in the fluid must be very small—indeed, the particles are generally considered to be contaminants. It is important to distinguish the science of particle counting from other scientific fields, such as photometry and cytometry, which also utilize scattered light, but in which the density of the particles in the fluid is relatively large; often it is the particles of the fluid itself that are detected and analyzed. These latter systems rely on collecting scattered light from thousands, millions, and even billions of particles; therefore, their principles of operation are very different from the principles used in particle counters.
Particle counters are generally used to detect contaminants in extremely pure fluids, such as those used in high tech electronics and the pharmaceutical industry. Generally, small samples of the fluids used in the manufacturing processes are diverted to the particle counters, which sound an alarm if the number and/or size of the particles detected is above a predetermined threshold. Since a small sample of the manufacturing fluid is generally not completely representative of the entire volume of the manufacturing fluid, statistics is used to extrapolate the state of the manufacturing fluid from the sample. The larger the sample, the more representative it is, and the more quickly an accurate determination of the number and size of particles in the manufacturing fluid can be made. Thus, It is desirable for a particle counter to detect particles as small as possible, as fast as possible, in as large a sample as possible.
Physical constraints require tradeoffs between the above goals. For example, sample volume and speed usually must be sacrificed to detect smaller particles. This is a direct result of the fact that, for particles to be detected in a particular fluid, the fluid must be constrained to flow through the monitoring region of a particle counter. Physical objects, such as nozzles and flow tubes, must be used to direct the fluid flow to the particle counter monitoring region. If it is desired to detect the particles in the entire sample flow, then scattered light from the entire sample flow must be collected. This generally results in light scattered from the physical constraining objects, such as a nozzle or flow tube, also being collected, which light creates noise in the output. The noise prevents detection of extremely small particles. This noise can be avoided by detecting particles in only a small portion of the sample flow. Particle counters that attempt to count all the particles in a fluid sample are generally referred to as volumetric particle counters, and particle counters that detect particles in only a small portion of the fluid flow are generally referred to as in-situ particle counters.
The word in-situ in Latin literally means in the natural state. That is, ideally, it refers to measurements unaffected by the measurement instrumentation. In an in-situ system, to be unaffected from the constraining elements, the detected particles must be far from the constraining elements, and only particles in a small fraction of the sample fluid flow are detected. In-situ systems commonly process 5% or less of the sampled fluid. An in-situ single pass particle counter is disclosed in U.S. Pat. No. 5,459,569, issued Oct. 17, 1995 to Knollenberg et al., which patent is hereby incorporated by reference. As a result of measuring only a selected fraction of fluid flow, however, in-situ systems take more time to achieve a statistically significant determination of the fluid cleanliness level or fluid quality. When measuring particle contamination levels in a clean room environment, this extended measurement time generally incurs the risk that an unacceptably high level of airborne or liquid particle concentration could go undetected for substantial time periods, thereby allowing a large number of manufactured parts to be produced under unacceptably “dirty” conditions. This situation can lead to substantial economic loss owing to the waste of time and production materials in the affected facility.
Since it is practically impossible to actually measure 100% of the particles carried by flowing fluid, herein the term “volumetric” generally corresponds to systems which measure 90% or more of the particles flowing through a measurement device. Volumetric particle measurement systems generally provide the advantage of measuring a greater volume of fluid, whether liquid or gas, within a fixed time period, thereby enabling a more rapid determination of a statistically significant measure of fluid quality. In the case where the particle concentration exceeds a predetermined permissible limit, this more rapid fluid processing generally enables a defective manufacturing process to be halted more quickly and more economically than would be possible employing in-situ measurement systems. However, as indicated above, volumetric measurement systems generally experience more noise than do in-situ systems because the efforts expended to control the location and flow characteristics of the fluid being analyzed generally perturbs the characteristics being measured to a greater extent than does in-situ measurement. One example of a trade-off between measurement completeness and interference with measurement data is that which arises when establishing the proximity of placement of a fluid inlet nozzle to a laser beam. Generally, both the completeness of the measurement, i.e., the percentage of sample flow measured and the interference with this measurement, increase with increasing proximity of the nozzle to the laser beam.
In various circumstances, there may be measurement processes having characteristics which are intermediate between in-situ and volumetric processes. Thus, where in-situ measurement generally corresponds to particle measurement within 5% or less of fluid transported through a measurement device, and volumetric measurement generally corresponds to analysis of 90% or more of such fluid, it will be recognized that measurement processes may be configured to process 10%, 30%, 50%, or other percentages in between the levels associated with in-situ and volumetric operation. Accordingly, herein, the term “non-in-situ” measurement generally corresponds to measurement of a proportion of fluid equal to more than 5% of total fluid flow.
In the field of particle counting, the use of high power illumination generally enhances particle detection. Specifically, higher power levels generally enable the detection of smaller particles than lower power systems. Higher power levels also generally permit particles of a given size to be detected more quickly. Thus, lasers are generally used as the light source in particle counters. Laser particle counters are of two types: intracavity particle counters in which the sample volume passes through the laser cavity, and extracavity particle counters, usually referred to as “single pass” particle counters, in which the sample volume is located outside the laser cavity. Locating particle-containing fluid flow within the cavity of the laser illuminating the particles provides for higher illumination power levels than are available in single pass laser systems, because, to maintain the lasing action, only a limited amount of optical energy is allowed to pass out of the cavity. A state-of-the-art in-cavity laser particle measurement system is disclosed in U.S. Pat. No. 5,889,589, issued Mar. 30, 1999 to Jon C. Sandberg, which patent is hereby incorporated by reference herein. However, in such particle counters, significant fluid flow through the laser cavity tends to modulate the characteristics of the laser cavity, thereby introducing undesired noise due to the medium, e.g., the air molecules. For this reason, fluid flow rates are commonly reduced when employing in-cavity systems to minimize the introduction of the cavity modulation-related noise. Conventional laser pumping cavities also have cavity power fluctuations greater than 30% short term and 50% long term caused by such things as thermal effects, air density changes, and particulate contamination of the laser cavity. Further, it is difficult to maintain calibration with such power level changes, because not all noise levels track linearly with power. This results in calibration errors occurring in most systems as the power level decreases. For this reason, cavity systems need to be purged regularly, and the systems need to be disassembled regularly to mechanically clean them. Locating a fluid flow containing particles for counting and measurement outside a laser cavity in a “single pass” laser system utilizing a solid-state laser diode generally avoids all of these problems and permits larger fluid samples to be monitored, but at the expense of much lower available laser power.
The power available in particle counters that utilize laser diodes to detect particles in fluid is also limited by multi-mode effects which necessarily arise in large laser diodes. The presence of multiple modes in a laser beam energy spectrum makes it extremely difficult to shape the beam. Further, in multiple mode light sources, light noise resulting from spontaneous emission occurs and is difficult to eliminate. Thus, all known commercial particle counters that utilize laser diodes to detect and measure particles in fluids have, up until now, been limited to single mode systems, typically the fundamental transverse electromagnetic mode, referred to as the TEM00 mode. As known in the art, the light from laser diodes can be limited to a single mode by limiting their size. However, limiting the size of the laser diodes also limits their power.
Accordingly, there is a need in the art for a particle counter system and method which provides high power illumination in a low noise environment and which produces a scattered light energy spectrum which is readily convertible into particle measurement data. Further, to accomplish this in a non-in-situ system would be highly advantageous.